Phase Alignment among Multiple Transmitters

ABSTRACT

Systems and methods for phase alignment among multiple transmitters are described. In some embodiments, a method may include creating a loop between an RF transmitter and an RF receiver; measuring a first DC signal on the I and Q paths of the RF receiver without inserting a DC signal in the I and Q paths of the RF transmitter; measuring a second DC signal on the I and Q paths of the RF receiver while inserting a non-zero DC signal in the I and Q paths of the RF transmitter; and calculating a relative phase difference between the RF transmitter and the RF receiver using the first and second DC signals.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 62/147,123 titled “PHASE ALIGNMENT AMONG MULTIPLE TRANSMITTERS”and filed on Apr. 14, 2015, which is incorporated by reference herein.

TECHNICAL FIELD

This specification is directed, in general, to telecommunications, and,more specifically, to systems and methods for phase alignment amongmultiple transmitters.

BACKGROUND

In radio frequency (RF) communications, coverage and capacity can beimproved through the use of spatial diversity or spatial multiplexing.For example, using spatial multiplexing, data rates can be increased bytransmitting independent information streams from different antennas butusing the same channel as defined by frequency and time resources.

These systems are referred to as multiple input multiple output (MIMO)systems. Typically, MIMO systems require complex controllers to controlboth the transmission and receiving elements of the mobile station andthe base station.

Multi-stream single user MIMO transmission has been proposed and formspart of Wideband Code Division Multiple Access (WCDMA), Third GenerationPartnership Project-Long Term Evolution (3GPP LTE), and WiMax systemstandards. In single user multiple input multiple output (SU-MIMO), aMIMO receiver with multiple antennas and receiving circuitry receivesthe multiple streams, separates the multiple streams and determines thetransmission symbols sent over each stream of the spatially multiplexeddata streams.

SUMMARY

Systems and methods for phase alignment among multiple transmitters aredescribed. In an illustrative, non-limiting embodiment, a method mayinclude creating a loop between an RF transmitter and an RF receiver;measuring a first DC signal on the I and Q paths of the RF receiverwithout inserting a DC signal in the I and Q paths of the RFtransmitter; measuring a second DC signal on the I and Q paths of the RFreceiver while inserting a non-zero DC signal in the I and Q paths ofthe RF transmitter; and calculating a relative phase difference betweenthe RF transmitter and the RF receiver using the first and second DCsignals.

The method may include prior to measuring the first or second DCsignals, calibrating the loop. For example, calibrating the loop mayinclude performing a loop DC cancelation procedure. Additionally oralternatively, calibrating the loop may include performing an I/Qimbalance compensation procedure.

In some cases, calibrating the loop may include: inserting a tone intothe transmitter; running a Quadrature Modulator Correction (QMC) filterto provide frequency-independent compensation near DC; and freezing theQMC filter. The tone may have a frequency of approximately 200 kHz. Thenon-zero DC signal may correspond to a gain index. The method mayinclude repeating the measuring and calculating operations for anothergain index. The method may also include storing a relative phasedifference for each different gain index. Also, the method may includerepeating the creating, measuring, and calculating operations foranother RF transmitter within a single communication device.

The first DC signal may be given by D_(Io) and D_(Qo), the second DCsignal may be given by D_(Ip) and D_(Qp), the non-zero DC signal may begiven by D_(TXI) and D_(TXQ), and the relative phase may be calculatedas: atan((D_(Qp)-D_(Qo))/(D_(Ip)-D_(Io)))-atan(D_(TXQ)/D_(TXI)).

In other illustrative, non-limiting embodiments, an electronic circuitmay include a processor; and a memory coupled to the controller, thememory having program instructions stored thereon that, upon executionby the processor, cause the processor to: create a loop between an RFtransmitter and an RF receiver; measure a first DC signal on the I and Qpaths of the RF receiver without inserting a DC signal in the I and Qpaths of the RF transmitter; measure a second DC signal on the I and Qpaths of the RF receiver while inserting a non-zero DC signal in the Iand Q paths of the RF transmitter; and calculate a relative phasedifference between the RF transmitter and the RF receiver using thefirst and second DC signals.

The program instructions, upon execution, may cause the processor to,prior to measuring the first or second DC signals, calibrate the loop atleast in part by performing a loop DC cancelation procedure. The programinstructions, may also cause the processor to, prior to measuring thefirst or second DC signals, calibrate the loop at least in part byperforming an I/Q imbalance compensation procedure.

Again, the first DC signal may be given by D_(Io) and D_(Qo), the secondDC signal may be given by D_(Ip) and D_(Qp), the non-zero DC signal maybe given by D_(TXI) and D_(TXQ), and the relative phase may becalculated as:atan((D_(Qp)-D_(Qo))/(D_(Ip)-D_(Io)))-atan(D_(TXQ)/D_(TXI)). Thenon-zero DC signal may correspond to a gain index, and the programinstructions, upon execution, may cause the processor to repeat themeasuring and calculating operations for other gain indexes, and tostore a relative phase difference for each different gain index.Moreover, the program instructions may further cause the processor torepeat the creating, measuring, and calculating operations for anotherRF transmitter within a single communication device.

In yet another illustrative, non-limiting embodiment a communicationdevice may include an electronic circuit, comprising: a processor; and amemory coupled to the controller, the memory having program instructionsstored thereon that, upon execution, cause the processor to: create aloop between an RF transmitter and an RF receiver; measure a first DCsignal on the I and Q paths of the RF receiver without inserting a DCsignal in the I and Q paths of the RF transmitter; measure a second DCsignal on the I and Q paths of the RF receiver while inserting anon-zero DC signal in the I and Q paths of the RF transmitter; andcalculate a relative phase difference between the RF transmitter and theRF receiver using the first and second DC signals; and a plurality ofantennas coupled to the plurality of RF transmitters.

The program instructions, upon execution, may cause the processor to,prior to measuring the first or second DC signals, calibrate the loop atleast in part by performing a loop DC cancelation procedure and an I/Qimbalance compensation procedure. Yet again, the first DC signal may begiven by D_(Io) and D_(Qo), the second DC signal may be given by D_(Ip)and D_(Qp), the non-zero DC signal may be given by D_(TXI) and D_(TXQ),and the relative phase may be calculated as:atan((D_(Qp)-D_(Qo))/(D_(Ip)-D_(Io)))-atan(D_(TXQ)/D_(TXI)).

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made to the accompanying drawings, wherein:

FIG. 1 is a block diagram illustrating an example of an RF communicationsystem, according to some embodiments.

FIG. 2 is a block diagram of an RF circuit according to someembodiments.

FIG. 3 is a flowchart illustrating an example of a method for phasealignment among multiple transmitters according to some embodiments.

FIG. 4 is a graph illustrating an example of phase estimation againstgain according to some embodiments.

DETAILED DESCRIPTION

The techniques of this disclosure now will be described more fullyhereinafter with reference to the accompanying drawings. The techniquesof this disclosure may, however, be embodied in many different forms andshould not be construed as limited to the embodiments set forth herein.Rather, these embodiments are provided so that this disclosure will bethorough and complete, and will fully convey the scope of the disclosureto a person of ordinary skill in the art. A person of ordinary skill inthe art may be able to use the various embodiments described herein.

FIG. 1 shows an example of Radio Frequency (RF) communication system100. It is noted that embodiments described herein may be applicable toa variety of wireless devices, and that wireless communications 151 mayuse network(s) implemented according to a variety of standards and theirevolution, such as, for example, WCDMA (Wideband Code Division MultipleAccess), 3GPP LTE (Long Term Evolution), WiMax (WorldwideInteroperability for Microwave Access), UMB (Ultra Mobile Broadband),CDMA (Code Division Multiple Access), 1×EV-DO (Evolution-DataOptimized), WLAN (Wireless Local Area Network), and UWB (Ultra-WideBand) receivers.

As illustrated, communication system 100 includes base station 101,which may be a node B (NB), an enhanced node B (eNB) or any accessserver suitable for enabling user equipment (UE) 201 to accesswirelessly a communication system. Base station (BS) 101 may transmit touser equipment 201 via wireless environment communications channel 151,which may be known as the downlink (DL), and user equipment (UE) 201 maytransmit to base station (BS) 101 via the wireless environmentcommunications channel 151, which may be known as the uplink (UL).

Base station 101 can comprise a processor 105 which may be configured tocontrol the operation of the receiver/transmitter circuitry 103.Processor 105 may be configured to run software stored in memory 106.

Memory 106 may be further configured to store data and/or information tobe transmitted and/or received. Memory 106 may further be used to storeconfiguration parameters used by the processor 105 in operating the basestation 101.

Transmitter/receiver circuitry 103 may be configured to operate as aconfigurable transmitter and/or receiver converting between radiofrequency signals of a specific protocol for transmission over (orreception via) the wireless environment and baseband digital signals.Transmitter/receiver circuitry 103 may be configured to use memory 106as a buffer for data and/or information to be transmitted over orreceived from wireless environment 151.

In addition, transmitter/receiver circuitry 103 may further beconfigured to be connected to at least one antenna for receiving andtransmitting the radio frequency signals over the wireless environmentto user equipment 201 ₁ and 201 ₂ (collectively, “user equipment 201”).In FIG. 1, base station 101 is shown comprising two antennas, firstantenna 107 ₁ and second antenna 107 ₂, both configured to transmit andreceive signals. In other embodiments, base station 101 may have moreantennas represented by the dotted antenna 107 _(m) in FIG. 1.

Base station 101 may be connected to other network elements viacommunications link 111. Communications link 111 may receive data to betransmitted to user equipment 201 via the downlink and it may transmitdata received from the user equipment 201 via the uplink. This data maycomprise data for all of the user equipment within the cell or wirelesscommunications range operated by base station 101. Communications link111 is shown as a wired link. However, it would be understood that thecommunications link may further be a wireless communications link.

Still referring to FIG. 1, there is shown two user equipment 201 ₁ and201 ₂ within the range of the base station 101. However, it would beunderstood that there may be more or fewer user equipment within rangeof base station 101. In some implementations, any user equipment may bea mobile station, or may include other apparatus or electronic devicesuitable for communication with base station 101. For example, the userequipment may be cell phones, tablets, or laptop computers suitable forwireless communication in the environment.

FIG. 1 shows in more detail first user equipment 201 ₁. First userequipment 201 ₁ may comprise processor 201 configured to control theoperation of receiver/transmitter circuitry 202/203. Processor 201 maybe configured to run software stored in memory 107. Processor 201 mayfurther control and operate any operation required to be carried out bythe user equipment such as operation of the user equipment display,audio and/or video encoding and decoding in order to reduce spectrumusage, etc.

Memory 107 may be further configured to store data and/or information tobe transmitted and/or received. Memory 107 may further be used to storeconfiguration parameters used by processor 201 in operating userequipment 201 ₁. Memory 107 may be solid state memory, optical memory(such as, for example, CD or DVD format data discs), magnetic memory(such as floppy or hard drives), or any media suitable for storing theprograms for operating the processors, configuration data ortransmission/reception data.

Transmitter/receiver circuitry 202/203 may be configured to operate as aconfigurable transmitter and/or receiver converting between radiofrequency signals of a specific protocol for transmission over (orreception via) the wireless environment and baseband digital signals.Transmitter/receiver circuitry 202/203 may be configured to use memory107 as a buffer for data to be transmitted over or received fromwireless environment 151.

Transmitter/receiver circuitry 202/203 may also be configured to beconnected to at least one antenna for receiving and transmitting theradio frequency signals over the wireless environment to base station101. In FIG. 1 each user equipment is shown comprising 2 antennas, UE201 ₁ has first first antenna 251 ₁₁ and second antenna 251 ₁₂, and UE201 ₂ has first antenna 251 ₂₁ and second antenna 251 ₂₂.

Although FIG. 1 and the examples described hereinafter describe the userequipment and the bases station as having a processor arranged to carryout the operations described below, it would be understood that inembodiments of this disclosure the respective processors may comprise asingle processor or a plurality of processors. Any combination of one ormore of the processors and other components in FIG. 1 may be implementedby one or more integrated circuits.

Some embodiments of this disclosure maybe used in the LTE-Advancedsystem which may be part of 3GPP LTE. However, it should be appreciatedthat this is by way of example only and these systems and methods may beused in other systems as well.

In system 100, performance and energy efficiency may be improved byusing spatial beamforming, which spreads the transmission energy overmultiple transmitters whose combined transmission energy is comparableto the transmission energy of single transmission scenario. Inbeamforming, the same signal is emitted from each of the transmitantennas with appropriate phase (and optionally gain) weighting suchthat the signal power is maximized at the receiver input. Benefits ofbeamforming include increasing the signal gain from constructivecombining and to reduce the multipath fading effect. However, successfulbeamforming relies on the proper coordination of multiple transmitters;especially in terms of signal phases. That is, phase alignment betweenmultiple transmitters is a prerequisite to a successful beamforming.

Conventionally, fairly complex logic has been used for correct alignmentfor coherent signal insertion and detection or synchronized datacaptures among systems running in a different clock domain. In contrast,systems and methods described below simplify the method for phasealignment and reduce or eliminate the need for more complex solutions.

In various embodiments, systems and methods disclosed herein may rely onDC insertion instead of a coherent tone. Irrespective of the insertionnode, a DC signal—e.g., a signal with a DC component of a knownamplitude and phase—may be inserted, and the phase offset estimation maybe had without the need for a synchronization between a transmitter anda receiver. Because processor 201 or the transceiver device can insert aDC signal with known amplitude and phase, the phase differences betweendifferent transmitters can be compared by calculating the phase of theestimated DC (DC on I channel and Q channel) in the feedback receiver.

Ordinarily, impairments such as DC offset, I/Q imbalance, and 2nd ordernon-linearity of the transmitter and receiver would degrade the accuracyof the DC-based phase estimation. To address these problems, however,systems and methods described herein may use on-chip calibration logicwhich equalizes the entire TX-to-RX loop before the DC-based phaseestimation. Also, a single receive path is used as a common feedbackpath, which eliminates any potential errors introduced when differentreceive paths are used for each transmitter.

FIG. 2 is a block diagram of RF circuit 200 according to someembodiments. Particularly, processor 201 is coupled to clock 204, to RFtransmitters 202A-N, and to RF receiver 203. In various implementations,however, any other number of transmitters and receivers may be used.

In normal operation, processor 201 may provide I and Q signals to eachof transmitters 202A-N to effect a beamforming technique, for example.These signals are processed by their respective transmitters and aretransmitted via antennas 251 ₁₁ and 251 ₁₂.

For instance, RF transmitter 202A may receive I and Q signals 205 fromprocessor 201, add a selected DC component 206 to signal 205 via adder207, and mix the output of adder 207 with the output of phase rotationcomponent 208 via mixer 209, which may be (or act as) a digital phaserotator. The output of mixer 209 is provided to TX digital path logics210, DAC 211, and filter 212, before it is mixed with oscillator signal213 by mixer 214 and amplified by amplifier 215. Output 216 fromtransmitter 202A is then provided to antenna 251 ₁₁, for example.

To ensure phase alignment among the various transmitters 202A-N, acalibration procedure may be performed. During this phase alignmentprocedure, which is described in more detail in connection with FIG. 3,processor 201 may configure or create a loop between each oftransmitters 202A-N and receiver 203, one at a time, by properly routingthe output of transmitters 202A-N to the input of receiver 203 whilebypassing the corresponding antenna(s) suing a suitable loop-backcircuitry (e.g., a multiplexer). Each time another loop is set up,another transmitter is coupled to the receive path and then calibrated.

In the case of transmitter 202A, for instance, output 216 is provided tothe input of amplifier 217 of receiver 203. The output of amplifier 217is provided to mixer 219, which mixes that signal 206 with anotheroscillator signal 218. The output of mixer 219 is provided to analogfilter/baseband amplifier 220 and ADC 221, RX digital path logic 222,loop calibration block 223, and DC estimation or measurement block 224.In some cases, the DC estimation operation performed by block 224 mayemploy a digital low-pass filter (LPF) or the like.

Output 225 of DC estimation or measurement block 224 is provided toprocessor 201, which can then use the results of the calibrationprocedure to modify I and Q signals sent to transmitters 202A-N in amanner that ensures phase alignment between them during a beamformingoperation.

FIG. 3 is a flowchart of an example of method 300 for phase alignmentamong multiple transmitters. In some embodiments, method 300 includescreating or configuring a loop between an RF transmitter (202A) and anRF receiver (203) at block 301, and beginning a pre-calibrationprocedure. In some case, the pre-calibration includes a loop DCcancelation procedure and/or an I/Q imbalance compensation procedureperformed by loop calibration block 223.

Still referring to pre-calibration, the following conditions may beassumed in an ideal case. First, no DC offset components are added bythe TX and RX data paths. Second, there is a 90 degree phase differencebetween I and Q. Therefore, a DC signal may be used to estimate by howmuch the phase is rotated as the DC signal propagates through a givenloop. And, if the TX channel is static, it is assumed that the phasedifference is caused by the RX channel. Because actual paths are notideal, however, the following procedure may be conducted to make anon-ideal path close to an ideal one.

First, with regard to loop DC cancellation, in terms of phaseestimation, no interest is placed how much of the DC offsets come fromthe TX path or the RX path. Hence, in some cases, this operation can beperformed digitally by cancelling total DC offset from the loop at RXoutput. As to, loop I/Q imbalance compensation, the composite I/Qimbalance of TX-to-RX RX loop may be measured and compensated together.This operation may be performed, for example, using a single tap I/Qmismatch compensation logic which estimates the frequency independentcomponents of I/Q mismatches (60 dB of IRR guarantees 0.1 degree errorbetween I and Q).

In this example, as part of the pre-calibration procedure, block 302inserts a tone into the transmitter, block 303 runs a QuadratureModulator Correction (QMC) filter (e.g., 1-tap) to providefrequency-independent compensation, and block 304 freezes the QMCfilter. In some implementations, the tone may have a frequency ofapproximately 200 kHz.

Then, blocks 305 through 307 are performed by DC estimation ormeasurement block 224 for each TX-to-RX loop, and for each gain settingor index of interest: block 305 measures the magnitudes (D_(Io), D_(Qo))of a first DC signal on the I and Q paths of the RF receiver,respectively, without inserting a DC signal in the I and Q paths of theRF transmitter; block 306 measures the magnitudes (D_(Ip), D_(Qp)) of asecond DC signal on the I and Q paths of the RF receiver, respectively,while inserting a non-zero DC signal (D_(TXI), D_(TXQ) in the I and Qpaths of the RF transmitter; and block 307 calculates calculating arelative phase difference between the RF transmitter and the RF receiverusing these measurements. By changing the gain index, the receiver RFgain is also changed (increased or decreased).

For example, in some embodiments, the relative phase difference for theselected transmitter may be given by:atan((D_(Qp)-D_(Qo))/(D_(Ip)-D_(Io)))-atan(D_(TXQ)/D_(TXI)), where theatan functions are calculated in 4-quadrants. Upon repetition, method300 may store the relative phase difference for a plurality of differentgains (given by different gain indexes) for each transmitter, and it maydo so for every transmitter 202A-N in the array. For each transmitter,the process of FIG. 3 is repeated after a new loop is created undercontrol of processor 201. The phase difference may then be used byprocessor 201 in order to provide I and Q signals to transmitters 202A-Nthat are phased aligned when they get to their respective antennas.

In sum, when the RF gain is changed, the impedance of RF signal path ischanged. This may result in the phase shift of the signal. To maintainor improve signal quality, it is desirable to limit the amount of phasechange to below 5 degrees (or some other selected threshold value) forany gain change. A way to offset the phase change induced by the gainchange is via calibration. To perform this operation, method 300 maycalibrate (on-chip) the amount of phase shift for each gain step.

FIG. 4 is a graph illustrating an example of phase estimation againstgain according to some embodiments. As shown, curve 402 shows therelative phase in degrees as a function of gain index for a particulartransmitter. Generally speaking, phase differences may be acceptable upto a certain threshold (e.g., 5 degrees). As shown, gain indexes 402fall within this range, but index 403 does not, and has been estimatedor measured to be off by approximately 46°. Therefore, when transmittingsignals in a normal configuration using gain index 403, processor 201may offset the I and/or Q signals sent to that particular transmitter by46°.

It should be understood that the various operations described herein,particularly in connection with FIG. 3, may be implemented by processingcircuitry or other hardware components. The order in which eachoperation of a given method is performed may be changed, and variouselements of the systems illustrated herein may be added, reordered,combined, omitted, modified, etc. It is intended that this disclosureembrace all such modifications and changes and, accordingly, the abovedescription should be regarded in an illustrative rather than arestrictive sense.

A person of ordinary skill in the art will appreciate that the variouscircuits depicted above are merely illustrative and is not intended tolimit the scope of the disclosure described herein. In particular, adevice or system configured to perform audio power limiting based onthermal modeling may include any combination of electronic componentsthat can perform the indicated operations. In addition, the operationsperformed by the illustrated components may, in some embodiments, beperformed by fewer components or distributed across additionalcomponents. Similarly, in other embodiments, the operations of some ofthe illustrated components may not be provided and/or other additionaloperations may be available. Accordingly, systems and methods describedherein may be implemented or executed with other circuit configurations.

It will be understood that various operations discussed herein may beexecuted simultaneously and/or sequentially. It will be furtherunderstood that each operation may be performed in any order and may beperformed once or repetitiously.

Many modifications and other embodiments come to mind to one skilled inthe art to which this disclosure pertains having the benefit of theteachings presented in the foregoing descriptions, and the associateddrawings. Therefore, it is to be understood that this disclosure is notto be limited to the specific embodiments disclosed. Although specificterms are employed herein, they are used in a generic and descriptivesense only and not for purposes of limitation.

Unless stated otherwise, terms such as “first” and “second” are used toarbitrarily distinguish between the elements such terms describe. Thus,these terms are not necessarily intended to indicate temporal or otherprioritization of such elements. The terms “coupled” or “operablycoupled” are defined as connected, although not necessarily directly,and not necessarily mechanically. The terms “a” and “an” are defined asone or more unless stated otherwise. The terms “comprise” (and any formof comprise, such as “comprises” and “comprising”), “have” (and any formof have, such as “has” and “having”), “include” (and any form ofinclude, such as “includes” and “including”) and “contain” (and any formof contain, such as “contains” and “containing”) are open-ended linkingverbs. As a result, a system, device, or apparatus that “comprises,”“has,” “includes” or “contains” one or more elements possesses those oneor more elements but is not limited to possessing only those one or moreelements. Similarly, a method or process that “comprises,” “has,”“includes” or “contains” one or more operations possesses those one ormore operations but is not limited to possessing only those one or moreoperations.

1. A method, comprising: creating a loop between an RF transmitter andan RF receiver; measuring a first DC signal on the I and Q paths of theRF receiver without inserting a DC signal in the I and Q paths of the RFtransmitter; measuring a second DC signal on the I and Q paths of the RFreceiver while inserting a non-zero DC signal in the I and Q paths ofthe RF transmitter; and calculating a relative phase difference betweenthe RF transmitter and the RF receiver using the first and second DCsignals.
 2. The method of claim 1, further comprising, prior tomeasuring the first or second DC signals, calibrating the loop.
 3. Themethod of claim 2, wherein calibrating the loop includes performing aloop DC cancelation procedure.
 4. The method of claim 2, whereincalibrating the loop includes performing an I/Q imbalance compensationprocedure.
 5. The method of claim 2, wherein calibrating the loopfurther comprises: inserting a tone into the transmitter; running aQuadrature Modulator Correction (QMC) filter to providefrequency-independent compensation near DC; and freezing the QMC filter.6. The method of claim 5, wherein the tone has a frequency ofapproximately 200 kHz.
 7. The method of claim 1, wherein the non-zero DCsignal corresponds to a gain index.
 8. The method of claim 1, furthercomprising repeating the measuring and calculating operations foranother gain index.
 9. The method of claim 8, further comprising storinga relative phase difference for each different gain index.
 10. Themethod of claim 1, further comprising repeating the creating, measuring,and calculating operations for another RF transmitter within a singlecommunication device.
 11. The method of claim 1, wherein the first DCsignal is given by D_(Io) in the I path and D_(Qo) in the Q path,wherein the second DC signal is given by D_(Ip) in the I path and D_(Qp)in the Q path, wherein the non-zero DC signal is given by D_(TXI) in theI path and D_(TXQ) in the Q path, and wherein the relative phasedifference is calculated as:atan((D_(Qp)-D_(Qo))/(D_(Ip)-D_(Io)))-atan(D_(TXQ)/D_(TXI)).
 12. Anelectronic circuit, comprising: a processor; and a memory coupled to thecontroller, the memory having program instructions stored thereon that,upon execution by the processor, cause the processor to: create a loopbetween an RF transmitter and an RF receiver; measure a first DC signalon the I and Q paths of the RF receiver without inserting a DC signal inthe I and Q paths of the RF transmitter; measure a second DC signal onthe I and Q paths of the RF receiver while inserting a non-zero DCsignal in the I and Q paths of the RF transmitter; and calculate arelative phase difference between the RF transmitter and the RF receiverusing the first and second DC signals.
 13. The electronic circuit ofclaim 12, wherein the program instructions, upon execution, furthercause the processor to, prior to measuring the first or second DCsignals, calibrate the loop at least in part by performing a loop DCcancelation procedure.
 14. The electronic circuit of claim 12, whereinthe program instructions, upon execution, further cause the processorto, prior to measuring the first or second DC signals, calibrate theloop at least in part by performing an I/Q imbalance compensationprocedure.
 15. The electronic circuit of claim 12, wherein the first DCsignal is given by D_(Io) in the I path and D_(Qo) in the Q path,wherein the second DC signal is given by D_(Ip) in the I path and D_(Qp)in the Q path, wherein the non-zero DC signal is given by D_(TXI) in theI path and D_(TXQ) in the Q path, and wherein the relative phasedifference is calculated as:atan((D_(Qp)-D_(Qo))/(D_(Ip)-D_(Io)))-atan(D_(TXQ)/D_(TXI)).
 16. Theelectronic circuit of claim 12, wherein the non-zero DC signalcorresponds to a gain index, and wherein the program instructions, uponexecution, further cause the processor to repeat the measuring andcalculating operations for other gain indexes, and to store a relativephase difference for each different gain index.
 17. The electroniccircuit of claim 12, wherein the program instructions, upon execution,further cause the processor to repeat the creating, measuring, andcalculating operations for another RF transmitter within a singlecommunication device.
 18. A communication device, comprising: anelectronic circuit, comprising: a processor; and a memory coupled to thecontroller, the memory having program instructions stored thereon that,upon execution, cause the processor to: create a loop between an RFtransmitter and an RF receiver; measure a first DC signal on the I and Qpaths of the RF receiver without inserting a DC signal in the I and Qpaths of the RF transmitter; measure a second DC signal on the I and Qpaths of the RF receiver while inserting a non-zero DC signal in the Iand Q paths of the RF transmitter; and calculate a relative phasedifference between the RF transmitter and the RF receiver using thefirst and second DC signals; and a plurality of antennas coupled to theplurality of RF transmitters.
 19. The communication device of claim 18,wherein the program instructions, upon execution, further cause theprocessor to, prior to measuring the first or second DC signals,calibrate the loop at least in part by performing a loop DC cancelationprocedure and an I/Q imbalance compensation procedure.
 20. Thecommunication device of claim 19, wherein the first DC signal is givenby D_(Io) in the I path and D_(Qo) in the Q path, wherein the second DCsignal is given by D_(Ip) in the I path and D_(Qp) in the Q path,wherein the non-zero DC signal is given by D_(TXI) in the I path andD_(TXQ) in the Q path, and wherein the relative phase difference iscalculated as:atan((D_(Qp)-D_(Qo))/(D_(Ip)-D_(Io)))-atan(D_(TXQ)/D_(TXI)).